Using a pressure/force sensitive element such as a diaphragm or cantilever in conjunction with a strain sensing element for measuring acceleration, force or pressure is known in the art.
Wire transducers consist of one or more wires stretched and connected between two or more points on a diaphragm. Displacement of the diaphragm stretches the wire elongating it while reducing its cross-sectional area, and accordingly, increasing its resistance to the flow of electrical current in a proportional relationship to the diaphragm displacement. Also known are transducers wherein strain gages are bonded to the diaphragm using adhesives such as epoxy. These strain gages are sometimes made of thin copper alloy foil which is etched to define a gage geometry during manufacture. The foil strain gage is bonded over its entire length to the diaphragm. Here again, deflection of the diaphragm causes a strain in the strain gage element which changes its cross-sectional area and, accordingly, changes its resistance to the flow of electrical current.
Strain gage transducers are generally less expensive and more rugged than the larger wire transducers. Both types of transducers, however, have low gage factors (change in electrical resistance with respect to changes in strain in the gage element) and both produce hysteresis errors affecting accuracy, when the gage is subject to periodic changes in strain.
Improvements are provided by piezoresistive transducers using a single-crystal semiconductor material, such as silicon doped with boron, for a strain gage element. The semiconductor strain gage is embedded in or bonded to the diaphragm and offers the advantage of high gage factor (as much as one hundred times more sensitive than metal strain gages) and small size.
Improvements have also been provided by selectively changing the thickness of the diaphragm so that it will consist of relatively thick islands and a relatively thick rim portion, separated by thin portions. Because the deflection of the diaphragm is primarily along the thin portions, the strain gages can be advantageously mounted across the thin portions in an area of maximum strain to provide increased gage sensitivity.
U.S. Pat. No. 4,093,933 to Wilner teaches a transducer structure having a pressure diaphragm composed of a nonmetallic material that has been sculptured by etching to form thick islands and a thick rim interconnected by thin sheet material. The thick portions are separated by the thin flexures in which the strain resulting from the deflections of the diaphragm is concentrated. Piezoresistive semiconductor sensors, similar to the type described in U.S. Pat. No. 3,351,880, are bonded on opposite sides of the thin grooves and electrical connectors are metallurgically bonded to the pads of the sensors so that the sensors may be connected in a Wheatstone bridge type circuit. The sculptured pressure diaphragm is desirable and advantageous because it allows mounting the solid-state strain gage elements across the points of maximum deflection which in essence mechanically amplifies the strain being sensed or measured as the diaphragm responds to forces. This structure is highly desirable because of its increased sensitivity when compared to structures wherein the strain gage element is mounted directly to the diaphragm surface along the entire length of the strain gage element.
Transducers formed from a single crystal of semiconductor material wherein the strain gages are an integral part of the semiconductor diaphragm are known in the art. These transducers offer the advantage of being small in size while having a high gage factor and are easier to manufacture. Also, the integral structure eliminates the need for an adhesive joint between the diaphragm and the strain gage. The adhesive joint is a disadvantage because it is not as stable as the remainder of the crystal structure and there can be relative movement, or creep, between the portions joined by the adhesive. However, these transducers, theoretically, lack the sensitivity of transducers with a sculptured diaphragm having piezoresistive semiconductors secured across slots in the diaphragm.
In U.S. Pat. No. 4,498,229, Wilner teaches an improved piezoresistive transducer and method for making same, wherein the gages are defined upon the substrate and subsequently etched from the material of the substrate. In this piezoresistive transducer the gage element is etched free of the substrate at its midportion and is integral with the substrate at its remote end portions. This piezoresistive transducer provides all the advantages of higher sensitivity provided by the sculptured pressure diaphragm and the freed gage structures described above and it also has the advantages of the one piece integral strain gage/diaphragm structures described above.
Although the single-crystal transducer with etch-freed piezoresistive gages has many advantages over previously known and used transducers, these gages are still connected by interconnections or conductors which are etched on the surface of the substrate, but not etch-freed, or by metallic connectors between the various freed gages. Interconnections etched into the substrate act as structural members stiffening the diaphragm and undesirably reducing its sensitivity to outside forces. In the alternative, the use of metallic connectors between the etch-freed gages introduces undesirable thermal expansion variations between the interconnections and the remainder of the transducer structure and the above-mentioned mechanical hysteresis problems inherent with the metallic structure.
Although the prior art discloses single-crystal piezoresistive transducers having etch-freed gages, there is still a need for a simple, straightforward, reliable, easily fabricated single-crystal semiconductor pressure sensitive element having at least one etch-freed piezoresistive strain gage and at least one etch-freed conductor wherein the strain gage, the conductor and the element are all formed of the same silicon crystal substrate.
In U.S. Pat. No. 4,737,473, Wilner teaches using a strain gage spaced apart from a hinged portion and separated or freed therefrom by grooves within a substantially planar substrate. However, such a substrate is not designed to detect changes in pressure.